1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device, and more particularly to a method of forming a silicon-on-insulator(SOI) substrate provided with an isolation layer.
2. Description of the Related Art
Due to the fast development in semiconductor device such as high integration and high performance, a semiconductor device using the SOI substrate instead of a silicon substrate made of bulk silicon, has been proposed. The SOI substrate has a stack structure of a base substrate as a means for supporting, a buried oxide layer, and a semiconductor layer in which a device is formed later. The semiconductor device formed on the SOI substrate has advantages such as a complete isolation of adjoining devices by the buried oxide layer and especially a reduced the parasitic capacitance and fast operation.
Conventionally, there have been used methods of forming SOI substrate, such as SIMOX(separation by implanted oxygen) and bonding method.
In the method of forming the SOI substrate according to the SIMOX, a silicon substrate is provided and oxygen ions are then ion-implanted at a predetermined depth from an upper surface of the silicon substrate. Afterward, the silicon substrate is annealed so that the ion-implanted oxygen ions react with silicon of the silicon substrate thereby forming the buried oxide layer to divide the silicon substrate into a base layer and a semiconductor layer. However, the SIMOX method has shortcomings that it is difficult to control thickness of the semiconductor layer in which a device is formed later, and a considerable amount of process time is required.
Referring to the method of forming the SOI substrate according to the bonding method, a base silicon layer as a supporting means is provided and a buried oxide layer is formed on the base silicon layer. Next, a semiconductor silicon layer in which a device is formed later, is bonded on the buried oxide layer, and the semiconductor silicon layer is back-polished according to the CMP process, so as to obtain a semiconductor layer having a desired thickness. According to the bonding method, it is easy to control thickness of the semiconductor layer and the bonding method shortens process time, compared to the SIMOX method. Moreover, since the isolation layer may be provided at the semiconductor layer during the manufacturing process of the SOI substrate, a manufacturing step of forming the isolation layer can be deleted from known semiconductor manufacturing processes.
FIGS. 1A to 1D are cross-sectional views showing conventional manufacturing steps of forming the SOI substrate according to a bonding method. Detailed descriptions of the manufacturing method will be discussed with reference to accompanying drawings.
Referring to FIG. 1A, a base silicon layer 1 as a means for supporting is provided and a first oxide layer 2 is then formed on the base silicon layer 1.
Referring to FIG. 1B, a semiconductor silicon layer 3 is provided and trenches 4 are then formed in an upper surface of the semiconductor silicon layer 3 according to a known etching process. Herein, the trench 4 has a depth similar to thickness of a semiconductor layer in which a device is formed later. A second oxide layer 5 is formed with a thickness sufficient to fill the trench 4 completely.
Referring to FIG. 1C, the base layer 1 and the semiconductor silicon layer 3 is bonded such that the first and second oxide layers 2, 5 are contacted with each other. Next, the base silicon layer 1 and the bonded semiconductor silicon layer 3 are annealed so that the bonding intensity therebetween is increased.
Referring to FIG. 1D, the semiconductor silicon layer 3 is back-polished according to the CMP process using the second oxide layer 5 filled in the trench 4 as a polishing stopper. As a result, an SOI substrate 10 comprising the base silicon layer 1, the buried oxide layer 7 formed on the base silicon layer 3, and a semiconductor layer 3a having an isolation layer and disposed on the buried oxide layer 7 is obtained.
In general, the property of a semiconductor device being formed in the SOI substrate depends mainly on the uniformity of semiconductor layer. Accordingly, what is most important is to obtain the uniformity of semiconductor layer in case the SOI substrate is formed according to the bonding method.
As described above, however, when the semiconductor silicon layer is polished so as to obtain a semiconductor layer by a thinning process after isolation layers are formed in the semiconductor silicon layer, there is occurred a dishing D at a surface of the semiconductor layer 3a as shown in FIG. 1D. As a result, thickness of the semiconductor layer 3a is not uniform owing to the occurrence of dishing D. Therefore, when a device is formed on the above semiconductor layer 3a, it is difficult to expect improvements in the device property.